Physical Sciences Division Research Highlights

Degradation Mechanisms Uncovered in Li-Ion Battery Electrolytes

Left: Cartoon illustrating concept of electron-beam-induced degradation of lithium-ion battery electrolytes under conditions similar to those during battery operation. A fluid cell is analyzed with a scanning transmission electron microscope, or STEM. Electrons in the solvent and other electron-beam-induced radical species will interact through secondary chemical reactions with a lithium salt and solvent. Right: STEM image showing electron beam-induced breakdown—the two large, dark particles—of lithium salt in an electrolyte mixture. Enlarge Image.

Results: A team led by Pacific Northwest National Laboratory has
uncovered information about high-demand batteries that could improve an
essential component impacting their performance and longevity. The scientists
characterized the stability and interconnected degradation mechanisms in
electrolytes commonly used for lithium-ion, or Li-ion, batteries. They obtained
detailed chemical imaging data using an environmental liquid stage in a
scanning transmission electron microscope (STEM).

Why It Matters: To develop new battery technologies, novel electrolytes with
increased electrochemical stability are needed, preferably solid electrolytes
such as inorganic or salt complexes. Finding these electrolytes requires non-invasive
tools that can be used in situ at the active particle size level-the
nanoscale-to observe the processes that occur during battery operation. In this
study, the researchers used STEM.

"Currently, STEM is the
only experimental technique that gives information at the nanoscale during the
operation of lithium-ion batteries," said Dr. Nigel Browning, Chief
Science Officer for PNNL's Chemical
Imaging Initiative.
"The in situ liquid stage in a STEM allows the reactions inside a battery to be
characterized in real time. This study is a proof of principle of the STEM
approach that avoids the standard post-mortem analysis of lithium electrolyte
breakdown products."

The detailed
characterization offered by liquid-stage STEM can provide unique insights into
electrolyte behavior, either for use in future in situ battery studies or to
test new electrolytes, winnowing the library of candidate solutions for further
characterization and reducing the experimental time spent on less effective
electrolytes.

Methods: In their study, the scientists explored the stability of
five different electrolytes commonly used for Li-ion and LiO2
battery applications: three that contained lithium hexafluoroarsenate salt, one
containing lithium hexafluorophosphate, and one containing lithium triflate.

The researchers placed
miniature environmental chambers with different electrolytes in the path of the
STEM's electron beam. By allowing the electrolytes to be examined in a liquid
state, even when inserted into the high vacuum of the microscope, these
chambers simulated what is found inside an actual battery. Then, the electron beam
caused a localized electrochemical reaction inside the liquid cell that sped up
electrolyte degradation-the breakdown of a range of inorganic/salt complexes. The
microscope acquired real-time images with nanoscale resolution, showing the
earliest stages of damage nucleation.

The scientists also used electron
energy loss spectroscopy to verify the presence of the electrolyte and measure
other experimental parameters.

"Each electrolyte behaved
differently in the analysis," said Dr. Patricia Abellan, a PNNL postdoctoral
fellow and materials scientist. "The stability of the electrolytes investigated
here correlates with electrochemical trends reported in the literature, which
suggests that this technique could potentially give new insights into the
reduction and degradation processes that take place during the operation of
lithium-ion batteries."

What's Next? Once the
effect of the imaging electrons is fully calibrated, this approach could
potentially provide insights into the degradation mechanisms that occur during
the first stages of solid electrolyte interphase, or SEI, formation, which
electrically insulates the electrolyte and prevents further deterioration.

"One day in the near
future, in situ STEM could be used to study different processes through direct
visualization and in real time," Abellan said. "We could use it to optimize
current state-of-the-art and next-generation electrolytes."

Acknowledgments:

Sponsors: This work was supported by
PNNL's Chemical Imaging Initiative; the Joint Center for Energy Storage
Research (JCESR), an Energy Innovation Hub funded by DOE Office of Science,
Basic Energy Sciences; the National Science Foundation; and Florida State
University.